Switching techniques



sePL 14, 1965 G. s. s'ruBBs 3,206,621

SWITCHING TECHNIQUES Sept 14, 1965 G. s. sruBBs 3,206,621

SWITCHING TECHNIQUES SWITCHING TECHNIQUES Filed April 14, 1959 4Sheets-Sheet 4 Fig. 95?

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United States Patent O 3,206,621 SWITCHING TECHNIQUES Gilbert S. Stubbs,Levittown, Fa., assigner' to The Franklin Institute, Philadelphia, Ia.,a corporation of Pennsylvania Filed Apr. 14, 1959, Ser. No. 806,238Claims. (Cl. 36W- 149) This invention relates to switching techniquesfor varying the apparent values of fixed electrical parameters. Thesetechniques are particularly useful but not necessarily limited to use inanalog computers wherein circuit elements simulate analogous physicalconditions which may vary from time to time. The use of the switchingtechniques makes possible apparent change in the circuit parameter sothat variation in the physical condition may be simulated by variationin the circuit parameter.

In the copending U.S. application Serial No. 781,259, filed December 18,1958, now Pat. No. 3,011,316, my co-invention with George P. Wachtell,there is described an analog computer in which heat capacity issimulated by electrical capacitance and heat flow impedance is simulatedby electrical resistance. Under actual working conditions, the effectiveheat capacity, particularly that of the coolant, may change from time totime and the heat liow impedance may be materially modified by suchthings as surface iilrns. In simulating nuclear `reactors and heatexchangers where these factors become of substantial importance, it isnecessary to have a means whereby the analog parameters in the computercan either actually or apparently be varied. The present inventionprovides the means for varying the apparent values of fixed-valueparameters without material modification of the general arrange-ment ofthe computer circuitry.

The techniques involved in accordance with the present invention, forexample, permit the simulation of a resistance whose value can bemodified at will. A similar effect is possible using capacitance.Another possible effect is the simulation of a variable current source.In each case, the control is accomplished by use of a pulse train whichprovides a switching effect upon the circuit involved. In certaininstances, a particular type of switch has been found particularlyadvantageous but the invention is not limited to any one type ofswitching element.

In accordance with the method of the present invention and in allcircuits of the present invention, it is highly desirable that the pulserepetition time be sufficiently small in comparison with the responsetimes of the analog circuit in which the parameter is located that theeffect appears to be steady. In the usual situation, an averaging isachieved which gives the effect of a fixed value of the parameter forany given pulse train. For example, a resistor which is periodicallyconnected and disconnected to a voltage source produces a lower averagecurrent in the resistor than would occur if it were permanentlyattached. The effective resistive load on the voltage source is of avalue higher than the fixed resistor. By changing the ratio of the timeconnected to the voltage source to the time disconnected from thevoltage source, the effective resistance can he changed. The effectiveincremental capacitance of a fixed capacitor may also be adjusted by theuse of a periodic switching technique. Additionally, the effectivecurrent from a current source may be modified by a periodic switchingtechnique.

For a better understanding of the present invention, reference is madeto the following schematic circuit diagrams and voltage and currentdiagrams, in which FIG. 1 illustrates a section of an analog computercircuit in which one form of the present invention is employed;

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FIG. la shows a voltage pulse train for actuation of one of the switchesshown in FIG. l;

FIG. 2 illustrates a circuit for simulating variable resistance;

FIG. 2n shows the voltage applied at the terminals of the circuit ofFIG. 2;

FIG. 2b shows the switching voltage employed to actuate the switch andthe circuit of FIG. 2;

FIG. 2c shows the resulting current output pulses from which is derivedaverage current in the circuit of FIG. 2;

FIG. 3 illustrates a preferred form of switching device useful with thepresent invention;

FIG. 3a illustrates a wave-form for application to the positive terminalof the network of FIG. 3;

FIG. 3b shows a waveform for application to the negative terminal ofFIG. 3;

FIG. 4 illustrates an alternative arrangement to the circuit of FIG 3wherein electron tubes are substituted for semi-conductor diodes;

FIG. 5 illustrates a circuit for obtaining the variable current effect;

FIG. 5a illustrates a constant input voltage applied to the circuit ofFIG. 5;

FIG. 5b illustrates the voltage wave effect created by variable pulsewidth modulation oi the input voltage in the circuit of FIG. 5;

FIG. 5c represents the resulting effective current in branch a of thecircuit of FIG. 5;

FIG. 6 illustrates a circuit for simulating a Variable capacitance;

FIG. 6a shows a constant current signal which is injected at the inputterminal of the circuit of FIG. 6;

FIG. 6b shows a pulse train applied to actuate the switch of FIG. 6;

FIG. 6c illustrates the capacitor voltage with and without the pulsetrain;

FIG. 7 illustrates another variable capacitor circuit in which all theswitches are connected to ground or to a low impedance voltage source;

FIG. 7a shows the operating wave train for actuation of two oftheswitches of FIG. 7;

FIG. 7b shows the operating wave train applied to the other switch ofFIG. 7;

FIG. 8 illustrates a conventional capacitance charging circuit;

FIG. 8a is a voltage time diagram showing capacitor voltage versus timein the circuit of FIG. 8;

FIG. 9 illustrates the capacitor circuit of the present invention with aconstant voltage input;

FIG. 9a represents the pulse train for actuating the switch of FIG. 9;and

FIG. 9b represents the capacitor output voltage from the circuit of FIG.9.

Referring first to FIG. l, there is shown one portion of the computercircuit in the region denoted 10 or 10 in the above-mentioned copendingapplication of Gilbert S. Stubbs and George I. Wachtell. Here thecapacitor 10 simulates heat capacity of the heat exchanger or itsability to store heat. The resistances 11 and 12 simulate the flowimpedance between the heat exchanger coolants and the center of the heatexchanger wall. Numbers 10, 11 and 12 in this drawing represent thecorresponding parts of FIG. l of said application. The capacitor 13simulates the heat capacity of the wall or its ability to store heat. Inthe copending application the resistances R1 and R2 simulate the heatflow impedance between the heat exchanger coolants and the center of theheat exchanger wall. In the present application, the resistors I4 and 15in combination with switches 16 and 17 simulate variable heat tlowimpedance in the same regions as R1 and R2. Another switchingarrangement would permit variation of the effective capacitance of theheat exchanger coolants which in FIG. l of the copending application isrepresented by the fixed capacitors C1 and C2, here designatd 18 and 19.Switching elements 20 and 21 represent the switching by stepping relaysdescribed in the copending application. A typical wave form for theoperation of switches 16 and 17 is shown in FIG. la.

FIG. 2 represents a basic circuit for simulation of a variable resistor.In this circuit a fixed resistor 23 is connected in series with switch24 in a circuit across terminals 25 and 26. A voltage e, which isconstant relative to the wave form shown in FIG. 2b, is impressed acrossterminals 25 and 26. A voltage pulse train es, shown in FIG. 2b, isimpressed on switch 24 through suitable means 27 to effectuate alternateopening and closing of the switch. As a consequence, actual current inthe form of the pulses, shown in FIG. 2c, ows through resistor 23. Theresponse times of the circuits to which terminals 25 and 26 areconnected are sufficiently large in comparison with the pulse repetitiontime To that the current pulses in FIG. 2c are effectively averaged. Asa consequence of this averaging, the appearance to the circuit externalof terminals 25 and 26 `is that of a fixed resistance whose value isgreater than the actual resistance 23 and whose value is determined bythe ratio of the pulse width T to the pulse repetition time T0.

Examples of switches which can be used in FIGS. 1 and 2 are shown inFIGS. 3 and 4. The operation of these switches is known to the art.These switches present the particular advantage that they enable therelatively rapid switching required to effectuate the small pulserepetition times required.

Referring specifically to FIG. 3, the switch is constructed primarilyabout a bridge composed of four rectifiers 30, 31, 32 and 33 connectedat terminals 34, 35, 36 and 37. The terminals 34 and 35 are switchingterminals whereas the terminal 36 is connected to the circuit element tobe effected and the terminal 37 is preferably connected to ground orsome low impedance voltage source. The diodes are arranged so thatcurrent can flow from terminal 34 through diodes 30 and 31 to terminals37 and 36, respectively, and from terminals 36 and 37 through diodes 32and 33 to terminal 35. When the voltages esl and esg are of thepolarities indicated in FIG. 3 at terminals 38 and 39, the voltagesacross the diodes will be of the proper polarity to cause conduction. Ifthe resistances of the diodes as Well as the resistances 4t) and 41 areequal, or nearly equal, the diode bridge will be essentially at abalanced condition, in which case the voltage at terminal 36 would beessentially equal to the voltage at terminal 37. The diode bridge thusin effect forms a short circuit between terminal 36 and the groundterminal 37. When the voltages esl and esz applied to terminals 3S and39 are opposite to the polarities indicated in FIG. 3, the resultingvoltages on the diodes will be of such polarities to inhibit conduction.Under this condition the effective resistances of the diodes are so highthat terminal 36 may be considered as effectively disconnected from thesignal esl voltages and ground. The effective state of the diode switchis open The wave forms shown in FIG. 3a and FIG. 3b illustrate possibleswitching signals which may be applied to terminals 38 and 39,respectively, in order to obtain periodic switching effects employed inthis invention.

FIG. 4 illustrates another switching arrangement suitable for use inconnection with the present invention employing a pair of vacuum triodes43 and 44. The cathode of one tube is connected to the anode of theother in cach case at terminals 45 and 46. Terminal 46 is connectedground or to a low impedance voltage source, and terminal 45 isconnected to the circuitry in which the switch is employed. The grids ofthe tubes are connected to leads 47 and 48 upon which are impressedsimilar switching signals esl. When these signals are sufficientlypositive either of the tubes may conduct, and terminal 45 willeffectively be connected to terminal 46 and be at essentially the samepotential as terminal 46. When the signals impressed on the grid leads47 and 48 are not sufficient to produce conduction, however, terminals45 and 46 are essentially isolated from one another so that an openCircuit is effected.

In addition to electronic switches of the type described in FIGS. 3 and4, it is possible also to employ mechanical type switches whoseswitching time may be more limited than the electronic switches butwhich may be employed in circuits whose relative response times are muchgreater.

FIG. 5 illustrates ya circuit in which a variable current source issimulated. In this circuit, a trigger oscillator 5f) is used to actuatea variable pulse width circuit 51. Each trigger pulse from circuit 50causes circuit 51 to put out a Voltage pulse of Width T proportional tothe input signal el shown in FIG. 5a. The repetition of pulses in theoutput of circuit 51 produces the pulse train wave-form shown in FIG.5b. The wave-form in this instance has a maximum pulse voltagedesignated E0 which is large compared to the computing network voltageses and eb. The starting level of the pulses in e0 is well below thevoltages ea and eb of the computing network 52 in order that the diodes53 and 54 do not conduct in the absence of a pulse from the variablepulse width circuit. Voltage differences Eo--es and Ell-eb are impressedon the resistors Rs and Rb `respectively design-ated 55 and 56, duringthe time interval T of the voltage pulse. The voltages es and el, aresmall compared to the voltage E0 so that the currents fiowing in thepulse interval T are essentially independent of the computing networkvoltages. The wave-form of the current is flowing through resistor R,lis shown in FIG. 5c. This current which occurs in the form of pulses iseffectively average by the computing network whose response times arelarge compared to the pulse repetition time T0.

The circuit of FIG. 5 is capable of generating only positive values ofthe currents il, and is. However, it is possible to construct a circuitusing voltages of opposite polarity and diodes oppositely oriented togenerate negative currents. It is also possible to construct a circuithaving resistor-diode branches in which both positive and negativecurrents may be generated.

In passing, it may be observed that the circuit of FIG. 2 mayalternatively be employed to simulate a variable current source. Whenthis is done, terminal 25 may be considered as connected to a highvoltage source and terminal 26 connected to a computing network. Thehigh voltage source in this case would be constant rather than varyingas e0 does in FIG. 5b.

Referring to FIG. 6, a circuit for simulation of a variable incrementalcapacitance is illustrated. In this case, capacitors 59 and 64I,connected in parallel, constitute the fixed capacitance Whose effectivevalue is to be varied by switching technique using a single-poledouble-throw switch 61 in conjunction with the D.C. amplifier 62.Otherwise capacitor 59 takes no part in the variable capacitancesimulation except that its capacitance value is effectively added tothat of capacitor 60.

When the constant current wave-form of FIG. 6a is impressed on terminal63 and the switch position is varied in accordance with the pulse trainof FIG. 6b, :a waveform corresponding to the step-like wave of FIG. 6cis generated. A constant current source and with the switch at positionb, the wave-form is a constant-slope ramp as shown in FIG. 6c. The levelpositions of the step-like wave-form in FIG. 6c occur when the switch 61is in position a. When the switch is in this position, the voltage at 63is essentially clamped at the voltage level which existed at node 63 atthe instant before the switch was changed to position a. During the timethe switch is in position a, the voltage on capacitor 60 remainsessentially at its initial value because the amplifier connected to thecapacitor is designed to draw very little current at its input terminal.As a result of the unity gain of the amplifier, the voltage at terminala of the switch is always essentially equal to the voltage on capacitor60. The ramps in the step-like wave-form of FIG. 6c occur during thetime intervals that switch 61 is connected to terminal b. During thesetime intervals, the voltage ec increases at a constant rate determinedby the current i and the capacitors 59 and 60. In the case illustratedby FIG. 6, the effective capacitance when the switch is operated ishigher than the sum of capacitors 59 and 60 by a factor T a Tz-T1 Thevariable capacitor circuit of FIG. 7 is essentially identical in itsoverall operation to that of FIG. 6. The essential feature of FIG. 7 isthat it permits use of switching elements which can be connected only toa low impedance source or to ground. This variable capacitor circuitwould be particularly well suited to the use of the switches describedin FIGS. 3 and 4. The capacitor 65 plays a role similar to that ofcapacitor 59 in FIG. 6 in that it prevents the voltage at terminal 67from varying when the switch 68 is opened. The capacitor 66 plays a rolesimilar to that of capacitor 60 in the circuit of FIG. 6. Thiscapacitor, in combination with capacitor 65, determines the totalcapacitance at terminal 67 when the switches are in the positionsindicated in FIG. 7. Because of the requirement that the switches mustbe connected either to ground or to the low impedance output of anamplifier, the means for clamping the capacitor voltage employed in FIG.7 must be different from the means employed in FIG. 6. The clampingcircuit of FIG. 7 includes two D.C. amplifiers of unity gain, designated70 and 71, two switches, ldesignated 72 and 73, and a capacitor,designated 74. The inputs of the amplifiers 70 and 71 lare 4designatedas 7S and 76, and the outputs are designated respectively as 77 and 78.The clamping of the voltage at terminal 67 is initiated by openingswitches 68 and 72 and simultaneously closing of switch 73. Thisswitching action connects the output of amplifier 71 to terminal 67,effectively disconnects capacitor 66 so that it will not charge, anddisconnects capacitor 74 from the output of amplifier 77. At the instantof switching the Voltage of capacitor 74 is equal to the voltage ofterminal 67. This capacitor voltage working through amplifier 67 andswitch 73 effectively clamps the voltage at terminal 67. The switchingis effected as indicated by the pulse trains shown in FIGS. 7a and 7b.

Referring to FIG. 8, the circuit shown is a conventional circuitemploying a constant voltage source 80 to charge a capacitor 81 througha resistor 82 when switch 83 is closed. The wave-form which appears atterminals 84 and 85 of capacitor 81 as the charge on the capacitorincreases is shown in FIG. 8a.

The circuit of FIG. 9 is essentially the same as the circuit of FIG. 6except that the capacitor 81 is replaced by a variable capacitor circuitwhose effective capacitance is equal to that of capacitor 81. Theeffective capacitor circuit corresponds to the circuit of FIG. 6 andsimilar number designators are employed, with the addition of primesthereto, to indicate similar parts. Likewise, the circuit elementscorresponding to those of FIG. 8 are designed with similar designators,with the addition of primes thereto. The pulse train of FIG. 9arepresents the switching operation of switch 61', an the wave-form ofFIG. 9b represents the voltage at the terminals of the variablecapacitor circuit 81.

The specific embodiments of the present invention have been described insome detail and modifications thereof have been suggested. Additionalmodifications and similar circuits will occur to those skilled in theart. All such modifications and variations within the scope of theclaims are intended to be within the scope and spirit of the presentinvention.

I claim:

1. An analog computer having a circuit branch representing a variableresistance, said circuit branch comprising a circuit for simulating aresistance of variable size consisting of at least one fixed resistor inseries with a switch across a potential difference in the computercircuitry, said branch being analogous to representation by a fixedresistor of a specific larger resistance value in a specificapplication, means for sensing variation in some preselected variableparameter elsewhere in the computer and means responsive to the sensedvariations for oper ating the switch so that it opens and closes tointerrupt the flow of current through the resistor circuit branch andthereby presents a flow of current pulses to the computer such that theaverage current through the resistor is the same as it would be if theresistance itself had been varied to simulate a resistor of larger size.

2. An analog computer comprising a receiving network and a circuitbranch for simulating a variable current source comprising a variablepulse-train-modulated high voltage source connected in a series circuitwith a rectifier and resistor to the receiving network and meansresponsive to a variable parameter elsewhere in the cornputer andoperable on the variable high voltage source to produce variationstherein proportional to said variable.

3. The computer of claim 2 in which the interruption of current isachieved by changing the level of the output of the high voltage sourcesuch that the network voltage across the series resistor and diodecombination is of such a polarity that the diode would prevent currentiiow.

4. A variable capacitance simulating circuit for use in connection witha circuit requiring a variable capacitance comprising a unity gainamplifier, a fixed Value capacitor, a single pole double throw switchand a voltage source interconnected so that the pole of the switch isalternately connected to the capacitor terminal and the amplifier outputterminal, and the capacitor and the amplifier are connected together.

5. A variable capacitance simulating circuit for use in connection witha circuit requiring a variable capacitance comprising a fixed valuecapacitor, a switch between the capacitor and ground, two unity gainamplifiers, and amplifier whose input is connected to the capacitor andwhose output is connected through a switch to a second fixed valuecapacitor, a second amplifier whose input terminal is connected to thesecond capacitor and whose output terminal is connected through a switchto the first fixed value capacitor, wherein when the circuit is in onestate the first two switches are closed and the third is open and thepulse train changes the switches from this state to a second state wherethe first two switches are open and the third switch is closed.

References Cited by the Examiner UNITED STATES PATENTS 2,473,414 6/ 49Darlington 320-1 2,775,715 12/56 Tuttle 307-96 2,812,413 1l/57 Brown307-96 LLOYD MCCOLLUM, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent N6i3,206,621 september 14, 1965 Gilbert S. Stubbs It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 4, line 37, for "average" read averaged column 4, line 69, for"positions" read portions column 5, line 69, for "an" read and column 6,line 5l, for "and", second occurrence, read one --c Signed and sealedthis 24th day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

1. AN ANALOG COMPUTER HAVING A CIRCUIT BRANCH REPRESENSTING A VARIABLERESISTANCE, SAID CIRCUIT BRANCH COMPRISING A CIRCUIT FOR SIMULATING ARESISTANCE OF VARIABLE SIZE CONSISTING OF AT LEAST ONE FIXED RESISTOR INSERIES WITH A SWITCH ACROSS A POTENTIAL DIFFERENCE IN THE COMPUTERCIRCUITRY, SAID BRANCH BEING ANALOGOUS TO REPRESENTATION BY A FIXEDRESISTOR OF A SPECIFIC LARGER RESISTANCE VALUE IN A SPECIFICAPPLICATION, MEANS FOR SENSING VARIATION IN SOME PRESELECTED VARIABLEPARAMETER ELSEWHERE IN THE COMPUTER AND MEANS RESPONSIVE TO THE SENSEDVARIATIONS FOR OPERATING THE SWITCH SO THAT IT OPENS AND CLOSES TOINTERRUPT THE FLOW OF CURRENT THROUGH THE RESISTOR CIRCUIT BRANCH ANDTHEREBY PRESENTS A FLOW OF CURRENT PULSES TO THE COMPUTER SUCH THAT THEAVERAGE CURRENT THROUGH THE RESISTOR IS THE SAME AS IT WOULD BE IF THERESISTANCE ITSELF HAD BEEN VARIED TO SIMULATE A RESISTOR OF LARGER SIZE.